1. Field of the Invention
The present invention relates to inside-out (I/O) nuclear magnetic resonance (NMR) imaging techniques, and more particularly to a method for cancelling coherent ringing within an echo train and a method for phase encoded imaging under I/O conditions.
2. Description of the Related Art
In recent years, I/O NMR has become an increasingly important measurement technique for oilwell logging and has lately been applied also in other contexts like material research applications. Inside-out NMR is different from conventional NMR spectroscopy and imaging insofar as the investigated sample is outside the spectrometer, not in the center of a polarizing magnet. Therefore the applied magnetic fields, the static polarizing field and the oscillating or radio frequency (RF) field, are by no means homogeneous. From the point of view of the measurement the sample is infinite, and the spatial center of the volume, which contributes useful signal (henceforth called the resonance volume), is given by the points where xcfx89rf=xcex3B0. Here, xcfx89rf is the carrier frequency of the RF pulses, and xcex3 is the gyromagnetic ratio of the nuclei under investigation. The boundaries of the resonance volume are given roughly by the condition |B0xe2x88x92xcfx89rf/xcex3| less than B1. Therefore the spectral width of the signal that is measured in an I/O NMR experiment is always of the order of xcex3B1, except if it is reduced by using a frequency limiting detection filter. This means that frequency resolved information like chemical shifts cannot be obtained, and measurements are restricted to measurements of relaxation times, diffusion, flow, etc.
In logging applications, these measurements are made with (sets of) Carr-Purcell-Meiboom-Gill (CPMG) refocusing pulse sequences. The CPMG sequence invokes large off resonance and pulse flip angle error correction capabilities and is thus useful to conserve the signal in a large region surrounding the resonance condition. This characteristic is prerequisite for detecting the xe2x80x9ctruexe2x80x9d decay rate of the echo train;.otherwise the observed decay rate will be shortened and possibly dominated by a sequence dependent decay rate. Two CPMG measurements with inverted excitation pulse phases are generally combined to cancel NMR baseline effects and coherent ringing from the refocusing pulses. The result is what is called a phase alternated pair (PAP), disclosed in U.S. Pat. No. 5,023,551.
In the 1980""s, a class of multi-echo sequences was developed for NMR spectroscopy and imaging applications in which the phases of successive refocusing pulses are cycled. These are called phase cycled CPMG (PCCPMG) sequences. The group consists of two subgroups: sequences with a nucleus of 90 degree phase shifts between consecutive echoes (XY4, XY8, XY16), and the MLEV sequences with 180 degree phase shifts between consecutive echoes (MLEV4, MLEV8, MLEV16). Another CPMG type sequence, the Freeman Hill modification of CPMG (+x[xe2x88x92x+x]), performs under I/O NMR conditions almost exactly like CPMG. The PCCPMG sequences have an advantage over CPMG that the PCCPMG sequences conserve all components of the magnetization. They achieve this by canceling the spin rotations introduced by the preceding pulses by subsequent pulses within a cycle. The result is that the magnetization vector after a full cycle returns approximately to its original direction for a wide variety of conditions. By contrast, during CPMG sequences, the spin vector is always rotated around the save axis through the same angle. Thus, CPMG sequences conserve only one component of the transverse magnetization. The other component is rapidly uniformly distributed. Compared to CPMG, PCCPMG sequences conserve less signal bandwidth. Nevertheless, it has been shown that a PCCPMG sequence with 90 degree phase shift (XY16) can be used under I/O NMR conditions to measure T2 without spin locking effects that had been observed when using CPMG.
Recently, phase encoded imaging was demonstrated under I/O NMR conditions using a single echo sequence. True phase encoded imaging relies on the phase of the signal being proportional to the applied gradient. This is certainly true for the first echo under I/O conditions. It is not true for further CPMG echoes, since only one magnetization component is conserved.
There is room for improving NMR imaging techniques useful under I/O conditions.
Briefly, a method is provided for performing phase encoded inside-out magnetic resonance imaging. A static magnetic field is applied to a volume of an earth formation surrounding a borehole that polarizes the nuclear spin within the volume of earth formation. An excitation pulse is applied into the formation that rotates nuclear spin from a longitudinal axis of the static magnetic field to a plane transverse thereto. A sequence of refocusing pulses is applied a period of time after termination of the excitation pulse to generate a plurality of echoes. The echoes induced by the refocusing pulses are detected. Next, the magnetic field is altered such that for a period of time, the strength of the magnetic field in the volume is spatially dependent, thereby inducing a magnetic field gradient in the earth volume that advances the phase of the nuclear spin. The refocusing pulse sequence is run again after the phase advance to detect more echoes, and this process is repeated for further phase advances so that a refocusing sequence is run for each of several phase advances. The echoes detected for each refocusing sequence are added together and averaged. An image is then generated from the averaged echoes, such as by Fourier transform techniques. As a variation, the phase advance is applied after the excitation pulse and before the first refocusing pulse sequence. Several refocusing sequences are run with that phase advance. Then, after a waiting period to allow for repolarization, another excitation pulse is applied, and the process is repeated with another phase advance. It is also possible to combine these two phase processes, where the direction of the magnetic field gradients applied in each would be different.
In addition, a method is provided for eliminating ringing while measuring a nuclear magnetic resonance property of a volume of earth formation surrounding a borehole. An excitation pulse is applied into the formation that rotates the nuclear spin from a longitudinal axis of the static magnetic field to a plane transverse thereto. A sequence of refocusing pulses is applied a period of time after termination of the excitation pulse to generate a plurality of echoes. The phase of the refocusing pulses is changed so that pairs of echoes in the echo train have opposite ringing phase. The echoes are detected. Additional sequences of refocusing pulses are applied to generate more echoes. Echoes in the echo train having opposite ringing phase are added to cancel ringing in the echo train. The echo train can then be analyzed for amplitude and/or decay characteristics.